Novel low-melting salts with donor-acceptor substituents as targets for second-order nonlinear optical applications

Article abstract of DOI:10.1039/b810671a

We synthesized several novel low-melting ionic salts with donor-acceptor substituents and investigated their possible applications as second-order nonlinear optical materials. The Royal Society of Chemistry.

Full text of DOI:10.1039/b810671a

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Novel low-melting salts with donor–acceptor substituents as targets
for second-order nonlinear optical applicationsw
Zhi-Qiang Zhu,a Shaoji Xiang,a Qing-Yun Chen,a Chaosen Chen,b Zhuo Zeng,b
Yi-Ping Cui*c and Ji-Chang Xiao*a
Received (in Cambridge, UK) 24th June 2008, Accepted 11th July 2008
First published as an Advance Article on the web 1st September 2008
DOI: 10.1039/b810671a
We synthesized several novel low-melting ionic salts with donor–
acceptor substituents and investigated their possible applications as
second-order nonlinear optical materials.
melt-processability resulting from the low melting point and
high thermal stability of ionic liquid. Owing to these charac-
teristics, we reasoned that ionic liquids might be designed and
synthesized according to the structural requirements of or-
ganic NLO materials. Our earlier experiences11 in ionic liquids
prompted us to investigate the synthesis of novel low-melting
salts with donor–acceptor substituents and their potential
applications in nonlinear optics.
Organic nonlinear optical (NLO) materials have attracted
considerable attention due to their potential applications in
optical communication, data manipulation and information
processing.1 The main advantages of organic NLO materials
as compared with inorganic ones are their larger first hyper-
polarizabilities (b), shorter response time, lower dielectric
constant and better synthetic flexibility.2 Molecules of such
materials generally possess a large dissymmetric delocalized
p-electron system with a donor p-conjugated bridge acceptor
(D–p–A) structure. Current research activities are mainly
focused on optimizing the combination of donor and acceptor,
increasing conjugation length and improving molecular pla-
narity.3 However, widespread practical applications of organic
NLO materials still suffer from their low thermal stability and
poor processability.4
Imidazolium is one of the most frequently used cations in the
development of new ionic liquids because of its reported stability
to degradation.12 Imidazolium cations containing a conjugated
aromatic core were then chosen for study. Coupling reaction of
imidazole and aryl iodides was carried out according to the
method of Ma and co-workers13 to afford the corresponding
N-arylated products. Reaction with 1.2 equiv. of 1-chloro-2,4-
dinitrobenzene at 120 1C led to the quaternized salts 2a–d in
good yields (Scheme 1). Subsequent metathetical reactions with
potassium hexafluorophosphate (KPF6) or lithium bis(trifluoro-
methanesulfonyl)amide (LiNTf2) resulted in the formation of 3
and 4, which were fully characterized by NMR, IR, MS and
elemental analysis. The structure of 3d was further confirmed by
single-crystal X-ray diffraction analysis as shown in Fig. 1.z
Melting points determined by differential scanning calorimetry
(DSC) are given in Table 1. It can be seen that the anion exhibits a
strong influence on the melting point. With the same cations, the
respective meltingꢀpoints decreased greatly on changing the anion
The past few years have witnessed a growing interest in ionic
liquids composed entirely of organic cations and organic or
inorganic anions.5 They are salts with low melting points
(below 100 1C, often even lower than room temperature),
which have been widely used as alternative solvents in green
chemistry due to their unique physicochemical properties such
as negligible vapor pressure, large liquid ranges and environ-
mental benignancy. Most importantly, their properties could
be fine-tuned by choosing suitable combination of cation and
anion.6 It is possible to design an ionic liquid for a specific
application by the modification of the substituents on the
cation or anion. Recently, novel ionic liquids have been
designed and synthesized for materials application such as
energetic density materials,7 magnetorheological fluids,8 liquid
crystals,9 and so on.10 Compared with the existing non-ionic
materials, these ionic analogues demonstrated an improved
ꢀ
from Clꢀ to PF6 and NTf2 (Table 1). Salts 4b and 4d have
Tm of o100 1C and therefore may be considered ionic liquids
(entries 10–11). While most of the chlorides decomposed
before melting, the corresponding hexafluorophosphates or
bis(trifluoromethanesulfonyl)amides have excellent thermal
a Key Laboratory of Organofluorine Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin
Road, Shanghai, 200032, P. R. China.
E-mail: jchxiao@mail.sioc.ac.cn; Fax: (+86) 21-6416 6128
b College of Chemistry and Environment, South China Normal
University, Guangzhou, 510631, P. R. China
c Advanced Photonics Center, School of Electronic Science and
Engineering, Southeast University, Nanjing, 210096, P. R. China
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data. CCDC 682525. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b810671a
Scheme 1 General synthetic routes to 2, 3 and 4.
ꢁc
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anion. These results indicated that these ionic salts could
possess larger hyperpolarizabilities with stronger electron-
donating ability of the substituent on the phenyl ring.
In conclusion, we have successfully achieved the synthesis of
several novel low-melting salts with donor–acceptor sub-
stituents. Some of the salts fall into the ionic liquid class
(Tm o 100 1C). We have explored the possible application
of ionic liquids as second-order nonlinear optical materials.
Most of them exhibit excellent properties including good NLO
effect, high thermal stability and low melting point, which may
be useful for nonlinear optical applications.
Fig. 1 Single-crystal X-ray crystal structure of 3d.
Table 1 Thermal properties of 2, 3 and 4
We thank the Chinese Academy of Sciences (Hundreds of
Talents Program) and the National Natural Science Founda-
tion (20772147) for financial support.
Entry
Compd.
R1
Tma/1C
Tdb/1C
c
1
2
3
4
5
6
7
8
2a
2b
2c
2d
3a
3b
3c
3d
4a
4b
4d
b
H
CH3
OH
OCH3
H
CH3
OH
OCH3
H
CH3
OCH3
—
—
—
—
253.8
231.5
228.9
223.6
314.8
323.3
277.3
297.2
404.6
375.8
368.2
c
Notes and references
c
c
z Crystal data for 3d: C16H13F6N4O5P, M = 486.26, monoclinic,
space group P21/c,
V
a
=
=
11.1474(10),
b
=
=
16.3515(15),
Z
c
=
=
4,
203.2
161.3
215.3
159.4
109.3
69.6
12.3412(11) A,
2024.6(3) A3,
T
293(2) K,
m(Mo-Ka) = 0.228 mmꢀ1, R1 = 0.0755, wR2 = 0.2264 (I 4 2s(I));
R1 = 0.1069, wR2 = 0.2522 (all data). CCDC 682525.
9
10
11
a
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82.3
c
Melting point. Thermal decomposition temperature. Decom-
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stabilities even at around 300 1C as determined by thermogravi-
metric analysis (TGA) (entries 5–11).
With these low-melting salts in hand, the second-order
nonlinearities were then evaluated using a hyper-Rayleigh
scattering14 (HRS) measurement as shown in Table 2. The
first hyperpolarizabilities of these compounds were all larger
than those of PNA (4-nitroaniline), which indicated that they
might become better second-order NLO materials than PNA
derivatives. The relationship between the structure and the
hyperpolarizabilities could be clearly observed. On the one
hand, the anion has a certain influence on the hyperpolariz-
ability. For a given substituent on the phenyl ring, changing
the anion PF6ꢀ in 3a–c to NTf2ꢀ in 4a–d led to some decrease
in the second-order NLO properties. On the other hand,
comparison of hyperpolarizabilities of the ionic salts contain-
ing differently substituted cations with the same anion clearly
illustrates the influence of the substituents. For example, with
hexafluorophosphate (PF6ꢀ) as the anion, on variation of the
substituents from methyl (3b) to hydroxyl (3c), the value of
b (10ꢀ30 esu) increased from 176 to 214 (entries 2–3). A similar
tendency can also be observed from 4a to 4d with NTf2ꢀ as the
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Table 2 The second-order nonlinearities of 3 and 4
Entry
Compd.
R1
ba/10ꢀ30 esu
b0b/10ꢀ30 esu
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1
2
3
4
5
6
a
3a
3b
3c
4a
4b
4d
H
158
176
214
135
140
151
119
133
161
102
105
114
CH3
OH
H
CH3
OCH3
Measured in THF relative to PNA (bPNA = 21.4 ꢂ 10ꢀ30 esu) at
1064 nm, the error is ꢃ10%. Calculated using the two-level model.15
b
ꢁc
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ꢁc
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